Learn Finnish: Saimme koko olohuoneen tapetoiduksi eilen. - We got the whole living room wallpapered yesterday.

This is the core of the achievement construction you are currently learning. When you use saada (to get) + an object + a passive past participle in the translative case (-ksi), it means you managed to get something done or completed a task. Here, it emphasizes that you successfully finished wallpapering the room.

What is the difference in meaning between this sentence and just saying Tapetoimme koko olohuoneen eilen?

Saying tapetoimme (we wallpapered) simply states the fact that the action happened. Using the saada construction (saimme... tapetoiduksi) highlights the effort and the achievement. It translates best to 'we managed to get the living room wallpapered' or 'we got the living room done.'

Why is the object olohuoneen in the genitive-accusative case instead of the partitive?

The genitive-accusative case is used here because the action was fully completed. You didn't just wallpaper some of the living room; you got the entire task done. If the action were ongoing or incomplete, you would use the partitive case (olohuonetta).

Why doesn't the word koko take the same ending as olohuoneen?

Koko (whole/entire) is a special indeclinable adjective. This means it never changes its form to match the noun it modifies, regardless of what grammatical case the main noun is in.